Certain prior art relating to liquid level control systems was cited and discussed in our earlier application Ser. No. 627,518, filed Oct. 31, 1975, now U.S. Pat. No. 4,056,979. From these disclosures it is known in the art to provide a control system comprising a plurality of reed switches disposed spaced apart to be actuated by a magnet conveyed by a carrier moveable along a path adjacent the switches. The following United States Patents are representative of such teaching:
U.S. Pat. No. 3,198,902
U.S. Pat. No. 3,200,645
U.S. Pat. No. 3,484,774
U.S. Pat. No. 3,634,794
U.S. Pat. No. 3,646,293
U.S. Pat. No. 3,678,750
U.S. Pat. No. 3,788,340
U.S. Pat. No. 3,826,139.
A problem common to each of these is that while the reed switch is closed as the carrier moves past, as for example to indicate carrier position, the switch opens as soon as the carrier magnet passes by the switch. Consequently, the only time a circuit will be completed through the reed switch is when the carrier magnet is sweeping it, and in order to be useful the reed switch must therefore be electrically connected to a memory system which is undisturbed by the switch opening as the carrier continues to move past. One available solution as shown in U.S. Pat. No. 3,826,139, involves the use of a guide tube having a plurality of floats, one for each reed switch, in a liquid level sensor. The various floats each require a stop located immediately above and below the switch to arrest the float upon activation of the switch. As the liquid continues to rise above the float, the float remains in the activating position until the level again drops. This system is costly because of the number of floats and stops required and appears limited in its application to liquid level sensing. The practicality of providing a number of magnet carriers along a machine control sensing system would be highly inefficient.
Another solution is proposed by U.S. Pat. No. 3,198,902 using a self-latching reed switch. However, in order for the switch to operate properly the magnet must bypass the reed switch through a specific sensing area shown in FIG. 3 of the patent. Thus the magnet and switch in U.S. Pat. No. 3,198,902 cannot revolve relative to each other imposing serious limitations on design flexibility. In the invention disclosed in the present application the actuating magnet and reed switches can revolve relative to each other, thus substantially increasing the design flexibility of the system in adapting it to various machine control environments.
The prior art illustrating systems using reed switches in combination with a memory system are generally unacceptable as creating problems during a power failure. If such a power failure occurs, the reed switch/memory system combination immediately loses track of the location of the magnet carrier when the power returns, whereas the present invention retains its pre-existing condition.